It is often desirable to position wells close together. For example, in certain oilfield applications, drilling wells close together may facilitate extraction of heavy oil. Indeed, heavy oil is generally too viscous in its natural state to be produced from a conventional well. Accordingly, to produce heavy oil, multiple wells, such as a pair of Steam Assisted Gravity Drainage (SAGD) wells, may be employed. A SAGD well pair typically includes two substantially parallel horizontal wells that are positioned relatively close together in a stacked orientation, and that use superheated steam to heat heavy oil until its viscosity is low enough to be produced. The upper well in a SAGD well pair may be referred to as an injector well. The lower well in a SAGD well pair may be referred to as a producer well. In operation, the injector well typically injects superheated steam into a heavy oil zone formation, creating a steam chamber to heat the heavy oil contained therewithin. When the heated heavy oil becomes less viscous, gravity pulls the oil into the producer well, which facilitates extraction of the oil.
When wells are drilled close to one another, there are typically spacing requirements that are essentially impossible to meet using conventional surveying techniques. For example, in a SAGD well pair, it may be desirable for the injector well and the producer well to be drilled such that they maintain a target separation distance, such as an approximately constant vertical separation distance (e.g., 4 to 6 m) over a horizontal distance (e.g., 500 m to 1500 m). Conventional measurement while drilling (MWD) survey data, which may only include inclination and azimuth data, does not provide sufficient accuracy to maintain a consistent separation distance between the injector well and the producer well. Accordingly, conventional spacing techniques may employ conventional magnetic ranging to facilitate drilling two or more wells within a specified distance from each other. For example, such conventional magnetic ranging techniques may be utilized to drill the second of the two wells of a SAGD well pair in an appropriate location with respect to the first well.
Conventional magnetic ranging techniques involve placing wireline equipment in a first well while a second well is drilled a specified distance from the first well. In operation, a magnetic field between the wireline equipment in the first well and a drilling bottom hole assembly (BHA) in the second well may enable the BHA in the second well to maintain an accurate vertical separation distance between the first and second wells of the SAGD pair. For example, the wireline equipment may include a solenoid configured to generate a known magnetic field, as is done with a Magnetic Guidance Tool (MGT), and the field may be detected to facilitate guiding the BHA. In another example, a wireline magnetometer may be used to measure a magnetic field generated by permanent magnets in the BHA of the second well, as is done with Rotating Magnet Ranging (RMR) systems, to facilitate guiding the BHA. However, when the wells are horizontal, these conventional magnetic ranging techniques require a wireline tractor to push the wireline tool down the length of the first horizontal well as the second well is being drilled. This can be an expensive procedure in a typical cased-hole tractor operation. Further, such a procedure may be even more difficult, expensive, and risky for an openhole tractor operation.